Design Patterns in Practice: Strategy, State, Observer, Builder, and Decorator Patterns

The article uses a teacher‑student dialogue to show how classic design patterns solve real‑world backend problems.

First, the reward distribution scenario demonstrates the Strategy pattern (encapsulating algorithms) combined with the Adapter pattern to decouple reward issuance from concrete service implementations. Example code: interface Strategy { void issue(Object... params); } and concrete strategies for Waimai, Hotel, Food.

Second, the task model design applies the State pattern to manage task state transitions and the Observer pattern to notify external services upon task completion, reducing coupling and satisfying the Open/Closed principle.

Third, the activity model is refactored using the Builder pattern for flexible object construction and the Decorator pattern to add risk‑control behavior without modifying existing participate methods.

Finally, the article concludes that mastering design patterns improves code maintainability, extensibility, and system design skills.